Support group milestone references

What does this MR do?

1. Support references for group milestones. (But only by name, and not cross-project references.)
2. Add a system note when a group milestone is set, using that reference.
3. Add group milestones to autocomplete when typing %.

As a consequence, quick actions now work for group milestones!

Are there points in the code the reviewer needs to double check?

The routing is a bit sketchy, but I don't see a nicer way to do it, other than having my new module extend self, and using that directly.

Why was this MR needed?

To make group milestones usable for us (i.e. me).

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests
  • Added for this feature/bug
  • All builds are passing
• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/34778.